That's right, the Flex SDK is going to be licensed under the Mozilla Public License - this means the compiler, debugger, the huge libraries - it's all going to be open source when Flex 3 "Moxie" is going to be released.
This news was not so surprising to me because I've heard there have been lots and lots of discussions before on how to make Flex "open". You have to keep in mind that a big company like Adobe is making transition from being a fully closed company to being more open - from the introduction of Adobe Labs to getting more open culture infused from Macromedia, to donating the ActionScript3 VM as open source to Mozilla, and now Flex. You can clearly see how this evolution is happening. Even the PDF format is going from a de-facto standard to a de-jure standard by the ISO organization.
Coming back to Flex, just some time ago I had noted how open Flex is, even source-open but not "open source", we're now taking the next big step and making even the code free. Awesome stuff.
I personally still think there's one issue that needs to be addressed. There are two parts to the story - the production and consumption, which is, the creation and the playback. The creation part is now not just open but open source! Why not make the playback part open? If we can allow anyone to create a player that plays SWF formats, that would be great. Of course, if we make the Flash Player itself open source, that would be mind-blowing, but I think the minimum that should be done is to allow other SWF players in the market. Not that anybody can beat Flash Player (it's very very hard)...
The news is still sinking in (it was announced just an hour ago, as of this writing). So, more coverage here: